Clarification on Korea and China

I had an opportunity to talk to Dusty a bit last night regarding the escalating tensions between North and South Korea, and it became clear that I'd been unclear on what I meant by referring to China as the wildcard in the situation... I will try to clear up what I think about China's role in Korea here.

North Korea is something of a vassal state to China.  Without China's support, North Korea would almost surely collapse.  I believe most of North Korea's energy and food come from across the Yalu River.  What, then, is North Korea to China?  Well, historically it was a military and ideological ally.  When the Korean War went against the North Koreans, China invaded Korea to push back UN mandated forces resulting in setbacks and a stalemate that would eventually define the DMZ as it is today.  North Korea, ideologically speaking, in the old days anyway, was another Communist country.  On paper, I suppose, both are still Communist, though I think neither really has much to do with practical Communism anymore.  In China, Communism started its decline when Deng Xiaoping declared that "to get rich is glorious."  You can't actually be rich if you're living in a Communist state.  I'm less clear when North Korea transitioned from a Communist state to it's current status as a cult-of-personality dictator built entirely around an almost religious fanaticism for Kim Jong Il (Or his father, Kim Il Sung, before him). 

So, what's in it for China today?  That's what I keep asking myself.  The modern post-Cold War geopolitical landscape sees a single superpower (the US) with maybe three in development (EU, Russia, China).  China is no longer a Communist country, by ideology, and it seems to me they've adopted a statist, state capitalist, dictatorship-- something not entirely unlike the facist regimes before World War II.  It remains to be seen whether state capitalism is a sustainable system, or if the Chinese government can maintain its stranglehold on liberty as their populace develops wealth... But the overriding theme in Chinese decision-making seems pointed at economic and technological development.  Given that, I can't really understand why they remain committed to the continued existence of North Korea.  I understand that North Korea creates a buffer between China and South Korea, a state with a sizable American military presence and a powerful military of its own.  I understand that there are still some ideological and historical reasons for China's continued support of North Korea (not least of which should be their characterization of the Korean War as a war to repel American aggression in China)...

But on the other hand, I have to think there's a lot of upside in it for China if the Koreas were to be unified under "southern" rule.  North Korea would quickly become a growth market opportunity for any investor interested in exploting what should be rapid development of North Koreans (see East German example).  It should create positive economic growth right on China's doorstep, and one that China could participate in if they played their cards right.  I think at some point, that calculation should outweigh the geopolitical benefits of a troublesome ally (who very notably has a negative economic impact on the region) standing between you an American influence.  I hope some portions of the Chinese government are beginning to think so, as I think that's the only long-term peaceful resolution to the divide on the Korean Peninsula.